Dirtiable inode bdi default != sb bdi btrfs

[Christoph Hellwig]

On Thu, Sep 30, 2010 at 01:38:07AM +0200, Jan Kara wrote:
> > No.  For one thing we don't need any exception for correctnes alone -
> > even the block device variant would work fine with the default case.
> Here I don't agree. If you don't have some kind of exception, sb->s_bdi
> for both "block" and "mtd_inodefs" filesystems points to
> noop_backing_dev_info and you get no writeback for that one. So it isn't
> just a performance issue but also a correctness one.

Indeed - for internal filesystems that require writeback the change
causes trouble if they haven't registered a s_bdi.  But for all user
visible filesystems that doesn't happen as we require s_bdi for
sync or even unmounts to work.